Topic 03.2 Flashcards
another name of combustion chamber
combustor
what does the combustion chamber contain?
the burning mixture of air and fuel
what must the combustion chamber generate?
maximum heat release at a substantially constant pressure
when the combustion chamber operates properly what will the turbine receive?
a uniformly expanded, heated, and accelerated stream of gas
combustion chambers must be designed to enable more _________________ with less ___________ of the atmosphere.
efficient use of fuel, pollution
air enters the combustion chamber at a ____________ rate than which it enters the intake of the engine
slightly lower
flame rate of kerosene
30 ft/s
what does a divergent duct between the compressor outlet and combustion chamber inlet do in terms of air velocity and pressure
decreased air velocity and increased air pressure
the air is divided after it exits the high pressure compressor into
primary, secondary, and tertiary airflows
what path does the air follow as it enters the flame tube?
- passes through the snout
- divided to go through the perforated flare
- then the swirl vanes
- into the primary zone
a region of lower velocity recirculation immediately downstream of the fuel spray nozzle
primary zone
stable combustion is achieved within this zone
primary zone
the primary air is about ____ of the total airflow exiting the high pressure compressor
0.2
the primary air is mixed, in a ratio of approximately _____ by weight, with the fuel, and burnt
0.6256944444444446
how is the primary air velocity further reduced after being reduced in the divergent duct?
by being passed through the flare and swirl vanes
why should primary air be further reduced before entering the combustion chamber?
to avoid extinguishing the flame
what starts the recirculation within the primary region?
the shape and the position of the flare and swirl vanes
the remaining 80% of the output of the high pressure compressor air goes into the __________________________
space between the flame tube and the air casing
from the 80% of the output about ______ is allowed into the flame tube and called secondary air
0.2
how does the secondary air enter the flame tube?
through the secondary air holes
what forms when the secondary air reacts with the primary air flowing through the swirling vanes?
toroidal vortex
the temperature of the gases in the primary zone reaches about __________________, which is far too hot, so the gases must be cooled before they can exit the combustion chamber
2,000 degrees Celsius
the remaining 60% of the air leaving the HP compressor, the tertiary air, is fed into the flame tube through ______________________ in the flame tube
corrugated joints and dilution air holes
what is the tertiary air used for?
to cool both the air casing and the gas exiting the chamber
a gas turbine engine would probably start quite readily with ________ igniter operating however, most gas turbine engines have ______ igniters
only one, two
because there are only two igniters, an _____________ has to pass the starting flame between the combustion chambers
inter-connector
the pressure in a combustion chamber which has a flame in it drives those flames into ___________ combustion chambers, where the mixture is unlit through the ______________ pipe work; thus, the flame is passed around all the combustion chambers
adjacent, inter-connector
what allows for the elongation of the combustion chamber due to expansion?
the sealing ring at the turbine end
why can’t the combustion chamber expand in the direction of the compressor?
because it is fixed at the compressor end by being bolted to it
the sealing ring allows the chamber to expand into the _________, which is the portion of the engine immediately preceding the nozzle guide vanes while maintaining a ______________
nozzle box, gas tight seal
types of combustion systems
- multiple combustion chamber system
- turbo-annular system
- annular combustion chamber system
the straight through flow multiple combustion chamber system was supplied with air by a ____________________
centrifugal compressor
type of combustion system used on some earlier types of axial flow engine, and is still in use on centrifugal compressor engines
multiple combustion chamber system
example of a multiple combustion chamber system that uses a centrifugal compressor
Rolls Royce Dart
the multiple combustion chamber system consists of a number of the __________________, which each consist of __________, which has its own ___________
individual combustion chambers, flame tube, air casing
in a multiple combustion chamber system, where are the combustion chambers located?
disposed around the engine just to the rear of the compressor section
in a multiple combustion chamber system, what is the primary air scoop?
snout
in a multiple combustion chamber system, what allows the drainage of excess fuel from the combustion chambers in the unlikely event of the engine failing to start
drain tubes
what is the result when the fuel is not removed before
the next attempt to start
excessively high gas temperatures in the turbine region and torching
a very dangerous jet of flame from the rear of the engine
torching
in a multiple combustion chamber system, what are the two means of getting rid of the fuel?
- fuel drain system
- evaporation of the remaining traces of fuel from the combustion chambers, the turbine, and the jet pipe
in a multiple combustion chamber system, how does the fuel drain system work?
- drain tubes connect the lowest part of each chamber to the next chamber below it
- once in the bottom chamber, excess fuel exits the engine
in a multiple combustion chamber system, how does the blow out cycle work?
- any remaining traces of fuel within the engine must be evaporated by motoring the engine over
- the engine is rotated for the time normally allocated to a full start cycle, but with the HP fuel cock shut, and the ignition system deselected
- air from the compressor will flow through the combustion chambers, the turbine, and the exhaust system to evaporate any fuel which still remains
also called the cannular or can-annular system
turbo-annular combustion chamber system
how does the turbo-annular combustion chamber system differ from the multiple combustion chamber system?
it does not have individual air casings for each of the flame tubes
if the turbo-annular system does not have individual air casings, where are the flame tubes located?
the flame tubes are fitted within an inner and outer air casing making it a more compact unit
how does the annular combustion chamber system differ from the other types of combustion chamber systems?
it only has one flame tube, which is contained by an inner and outer air casing
what are the advantages of the annular combustion chamber system?
- it can have a shorter length with the same diameter for the same power output
- no flame propagation problems
- less cooling air required
- high combustion efficiency
- improved pressure distribution to turbine
to obtain the maximum heat release from burning the mixture of fuel and air in the combustion chambers, the ________________________ ratio of air and fuel of _______ must be used
chemically correct or stoichiometric, 15:1
ratio of air fuel mixture by weight in the primary zone
0.6256944444444446
ratio of air fuel mixture when secondary and tertiary air is added
45:1 to 130:1
what happens to the pressure as gases pass through the combustion chamber?
a small loss in pressure varying from 3% to 8%
during normal engine running conditions, the ignition system is switched __________ as soon as the engine has attained _________________
off, self sustaining speed
example of engine operating conditions that require the use of the ignition system
ingestion of large quantities of water during take off from a heavily contaminated runway
smooth burning of the mixture, coupled with the ability to remain alight over a large range of air/fuel and air mass flows
combustion stability
where is it more difficult to start combustion than it is to sustain it once it has started?
outside the ignition loop
restarting the engine in the air while it is windmilling
relight
should the engine flame out at a high speed or altitude, it may be necessary to _______________ and/or ___________ before the engine can be successfully relit
slow down, descend
efficiency with which the combustor assembly extracts the potential heat actually contained in the fuel
combustion efficiency
at high power operating conditions, combustion efficiencies as great as _____ are achievable
0.99
at idle, combustion efficiency can still be as high as ____
0.95
a combustion section is typically located directly between the ____________ and _______________
compressor diffuser, turbine section
basic elements of combustion sections
- one or more combustion chambers (combustors)
- a fuel injection system
-nan ignition source - a fuel drainage system
primary function of the combustion section
to burn fuel/air mixture, thereby adding heat energy to the air
meters the appropriate amount of fuel through the fuel nozzles into the combustors
fuel injection system
where are fuel nozzles located?
in the combustion chamber case
fuel is delivered through the nozzles into the liners in a __________________ to ensure thorough mixing with the incoming air
finely atomized spray
a typical ignition source for gas turbine engines
high-energy capacitor discharge system
what is a high-energy capacitor discharge system composed of?
- an exciter unit
- two high-tension cables
- two spark igniters
a high-energy capacitor discharge system produces ____________ sparks per minute, resulting in a ______________ at the igniter electrodes
60 to 100, ball of fire
accomplishes the important task of draining the unburned fuel after engine shutdown
fuel drainage system
draining the unburned fuel helps to prevent ___________ in the fuel manifold, nozzles, and combustion chambers which are caused by _______________
gum deposits, fuel residue
major advantage of multiple can type/tubular combustion chamber
multiple can combustors can be individually removed for inspection
how many cans are in a typical multiple can type/tubular combustion chamber
8 or 10
consists of a housing and perforated inner liner, or basket which must be removed as a single unit for repair or replacement
annular or basket type combustion chamber
who invented the can-annular or turboannular combustion chamber?
Pratt & Whitney
characteristics of a good combustion chamber
- mix fuel and air effectively in the best ratio for good combustion
- burn the mixture as efficiently as possible
- cool the hot combustion gases to a temperature the turbine blades can tolerate
- distribute hot gases evenly to the turbine section
situation wherein an excessively high velocity airflow could literally blow the flame out of the engine/combustion chamber
flameout
two types of flameout
- lean die out
- rich blow out
usually occurs at high altitude where low engine speeds and low fuel pressure form a weak flame that can die out in a normal airflow
lean die out
occurs during rapid engine acceleration when an overly-rich mixture causes the fuel temperature to drop below the combustion temperature or when there is insufficient airflow to support combustion
rich blow out
the working of a turbine is roughly similar to that of an _________________ working in reverse
axial flow compressor
the first part of the turbine is the _______________, which directs the air axially onto the blades of a rotor section
nozzle guide vane
extracts energy from the hot gases that flow through it, and converts that energy into mechanical energy, to drive the compressor and gearbox connected to it
turbine
forms of energy available in the gas stream flowing through the turbine
- heat energy
- potential or pressure energy
- kinetic energy from the velocity of the gas stream
when the turbine extracts the energy to power the compressor, the drop in pressure energy occurs _____________________
- as it is converted to kinetic energy in the nozzle guide vanes
- as it is converted into mechanical energy in the turbine blades
the turbine stage consists of which two elements?
one row of stationary nozzle guide vanes and one row of rotating turbine blades
affords longer and longer blades to be fitted to each turbine stage moving backwards in the engine
divergent gas flow annulus
enables the velocity of the gas stream to be controlled as the gas expands to a larger volume available to it
divergent gas flow annulus
fitted in an attempt to minimize losses due to leakage across the turbine blade tips and it also reduces the vibration of the blades
blade shroud
the clearance between the blade tips and the turbine casing varies because of the different rates of __________________ of the materials involved
expansion and contraction
in some engines, an ______________ is used in the turbine casing area, to reduce gas leakage through this clearance between the blade tips and the turbine casing, but __________________ can be more effective throughout the flight cycle
abradable lining, active clearance control
a turbine which is not connected to the compressor, it is connected only either to the reduction gear for propeller
free turbine or a free power turbine
the fact that it is not connected to a compressor allows a free power turbine to seek its own ________________, rather than that of a compressor
optimum design speed
how to increase the power output of a turbine?
by increasing its diameter
detrimental effects when the diameter of a turbine is increased
- increase the drag factor due to larger area
- greater stresses due to greater centrifugal forces created
allows an increase in power output with a reduction in turbine diameter
an increase in the number of stages which comprise the turbine
the efficiency of a turbine blade increases as its rotational speed ___________
increases
the stresses in the turbine blade increase as blade speed ______________
increase
a high bypass ratio engine consists of ________ spools
three
spools of a high bypass ratio engine
- high pressure (HP) spool
- intermediate pressure (IP) spool
- low pressure (LP) spool
the high pressure (HP) spool, driven by the __________, which rotates at a relatively ____________
HP turbine, high speed
to the rear of the HP turbine is the ___________________, driving the IP compressor through a shaft which ______________ that of a HP turbine
intermediate pressure (IP) turbine, spins inside
the rearmost turbine is the ______________________ that drives the LP compressor, more commonly called the _______, through a shaft which runs inside the HP and IP shafts
low pressure (LP) turbine, fan
nozzle guide vanes are of _________ shape with the space between two nozzle guide vanes forming a _______________
airfoil, convergent duct
in the nozzle guide vanes, what converts some of the pressure energy in the gas stream into kinetic energy?
convergent duct
types of turbine blades
- impulse type
- reaction type
- impulse-reaction type
a type of turbine blade similar in action to a water wheel
impulse type
a type of turbine blade that rotates as a reaction to the lift they create as the gas stream flows over them
reaction type
mixture of two types of turbine blades
impulse-reaction type
the shape of the combination impulse-reaction blade changes from its _________ to its ______
base, tip
how is the shape change accomplished in an impulse-reaction blade?
by having a greater stagger angle at its tip than at its base
ensures that the gas flow does equal work along the length of the blade and enables the gas flow to enter the exhaust system with a uniform axial velocity
the blade twist caused by the difference in stagger angle at the tip and at the base
most commonly used method of fixing the blade to the disc
fir tree fixing
in fir tree fixing the blade is free in the serrations while the engine is ___________, but the _________________ imposed during operation holds it firmly in place
not rotating, centrifugal force
average energy loss in the turbine
0.08
breakdown of the energy loss in the turbine
- 3.5% from aerodynamic losses in the turbine blades
- 1.5% from aerodynamic losses in the nozzle guide vanes
- the rest is divided fairly equally between gas leakage over the blade tips and exhaust system losses
during normal engine operation, the rotational speed of the turbine may be such that the blade tips travel faster than ________________
1,500 feet per second
the temperature of the gases driving the turbine can reach as high as _________________
1,700 degrees Celsius
the speed of these gases comes close to the ________________ at temperatures as high as 1,700 degrees Celsius
speed of sound
a small turbine blade, weighing only 2 ounces when stationary, can exert a load along its length which exceeds _________ when rotating at its maximum speed which under tremendous heat, causes a phenomenon called __________
two tons, creep
the stretching of the metal of the blade, beyond its ability to reform back to its original length
creep
turbine blades of early gas turbine engines were made from _________________
high temperature steel
after the use of high temperature steel, turbine blades were then made from ______________
nickel based alloys
nickel based alloys in turbine blade construction was superseded by _________
super alloys
mixture of super alloys
- chromium
- cobalt
- nickel
- titanium
- tungsten
- carbon
blades manufactured from super alloys have a maximum temperature limit of approximately ___________________, or if the blades are cooled internally, ____________________
1,100 degrees Celsius, 1,425 degrees Celsius
traditional metal manufacturing processes produce a _________________, in the material. The boundaries of these create a weakness in the structure, and are usually the starting point of any __________
crystal lattice or grain, failure
in the search for even stronger materials, a procedure called __________________ is now being used in modern engines
single crystal casting
this process virtually eliminates corrosion and creates an extremely creep resistant blade
single crystal casting
four basic elements of the turbine section of a turbojet engine
- case
- stator
- shroud
- rotor
encloses the turbine rotor and stator assembly, giving either direct or indirect support to the stator elements
turbine casing
a typical case has _________ on both ends that provide a means of attaching the turbine section to the combustion section and the exhaust assembly
flanges
a stator element is most commonly referred to as the _____________, _______________, or the ________________
turbine nozzle, turbine guide vanes, nozzle diaphragm
located directly aft of the combustion section and immediately forward of the turbine wheel
turbine nozzle
its purpose is to collect the high energy airflow from the combustors and direct the flow to strike the turbine rotor at the appropriate angle
turbine nozzle
the turbine nozzle assembly consists of an ________________ that retains and surrounds the nozzle vanes
inner and outer shroud
the nozzle vanes are _________________ into the inner and outer shrouds and to allow for expansion, the inner or outer shroud ring is _________________
rigidly welded or riveted, cut into segments
the rotating elements of a turbine section consist of a ________ and a _______________
shaft, turbine rotor (wheel)
a dynamically balanced unit consisting of blades attached to a rotating disk
turbine wheel
the anchoring component for the turbine blades and is bolted or welded to the main shaft
turbine disk
a phenomena in which extreme stress on turbine blades may cause the turbine blades to grow in length due to severe centrifugal loads imposed by high rotational speeds
growth or creep
airfoil shaped components designed to extract the maximum amount of energy from the flow of hot gases
turbine blades
turbine blades fit __________ into a turbine disk when an engine is cold, but _______________ at normal operating temperatures
loosely, expand to fit tightly
in the turboprop engine, where the thrust is provided by the propeller, the turbine gas temperature and back pressure at the turbine are dictated by the design of the _____________
outlet nozzle
the temperature of the gases entering the exhaust system can be between ________ and _________
550°C, 850°C
if the exhaust system runs through the fuselage of the aircraft, then the fuselage must be protected from the very high temperatures of the gases which is done by allowing a ___________ between the jet pipe and the aircraft skin through which air is allowed to circulate and insulating the jet pipe with some form of ____________ sandwiched between thin layers of stainless steel
clearance, fibrous material
the gas velocity leaving the turbine is between _____________, around __________
750 – 1,250 ft/s, 0.5 Mach
although the shape of the jet pipe outer casing appears to be _____________ at the point where the gas leaves the turbine, the shape of the volume within the casing is in fact ____________
convergent, divergent
how is divergence made possible where the gas leaves the turbine when the jet pipe outer casing is convergent?
through the insertion of the exhaust cone
a conical shaped device positioned close up to the rear turbine disc rear face
exhaust cone
aside from helping to reduce the velocity of the gases leaving the turbine, the exhaust cone also prevents the ___________ flowing across the rear disc face of the turbine, further reducing ____________, and preventing ______________ of the disc
hot gases, disturbance, overheating
the rear turbine bearing is also supported inside the exhaust cone via ____________________, which are streamlined by fairings
turbine rear support struts
the ___________ also straighten out any residual whirl which could cause additional losses through generating turbulence in the jet pipe
fairings
the exhaust gases exit to atmosphere via the __________________ which increases the velocity to speeds of ___________ in a turbojet engine at virtually all throttle openings above idle
convergent propelling nozzle, Mach 1.0
although the gas velocity and temperature exiting the engine are extremely high, the levels of both drop quite dramatically with ________________ from the exhaust nozzle
increasing distance
the _______________ combines the bypass air and the hot exhaust gases in a mixer unit
low bypass exhaust
ensures thorough mixing of the two streams before they are ejected into the atmosphere
mixer unit
two methods used to exhaust the cold bypass air and the hot exhaust gases
- the standard method where the hot and cold nozzles are co-axial and the two streams mix externally
- by fitting an integrated exhaust nozzle where the two gas flows are partially mixed together before their ejection into the atmosphere
the main source of noise emanating from an aircraft is generated by the _________
engine
the most significant sources of noise from the engine originate from the _____________, the __________, and the ___________
compressor, turbine, exhaust
the level of noise from the exhaust is more affected by a _______________, than the noise levels of either the compressor or the turbine
reduction in its velocity
the relative speed difference between the exhaust jet and the atmosphere into which it is thrusting causes a _________________ which creates a violent and extremely turbulent mixing pattern
sharp shearing action
where the turbulence zone is narrow, the _____________ noise is being generated in the mixing pattern, and when the turbulence zone is widened, _____________ noise is generated
high frequency, low frequency
components comprising the exhaust section
- exhaust cone
- exhaust duct or tailpipe
- exhaust nozzle
an extension of the exhaust section that directs exhaust gases safely from the exhaust cone to the exhaust, or jet nozzle
tailpipe
two types of exhaust nozzle designs
- converging design
- converging-diverging design
